Eastshade

In order to unlock this, you must reach the cave in Eastshade. Here's a video that might help:
(Keep in mind that this video is a walkthrough of the whole game. You unlock the achievement at around 6:34 in the video.)

This will take less than 5 minutes. Leave the sleeping quarters tern left and go to the old chimpanzee in front of you.
She will ask you to get her book from the sleeping quarters, get it and return it to her this will then allow topics to be chosen in future conversations. Speak to the other chimpanzee . Once this conversation ends the boat will start flooding and it will fade to black there are a few questions about your father trip to east shade. Then you will wake up in the cave and the achievement will unlock.